Several techniques have been used successfully and commercially heretofore to distribute hydrophobic compounds, particularly non-polymeric compounds such as color-forming couplers, ultraviolet absorbing materials and the like, fairly uniformly through layers of gelatin or other hydrophilic colloids in the manufacture of photographic products. One of the simplest of these techniques involves mechanically dispersing (i) the hydrophobic compound(s) in solid or liquid form through (ii) an appropriate hydrophilic colloid solution or emulsion by first blending together (i) and (ii), and then passing the resulting blend several times through a high energy mill such as a colloid mill. This technique produces inferior dispersions (as compared to other conventional dispersion techniques), which inferior dispersions are often unstable. Also large amounts of energy are consumed by this technique to accomplish the desired degree of particle comminution and dispersion. The large consumption of energy is often accompanied by heat buildup or undesirable localized heating and by undesirable chemical degradation of some of the ingredients involved.
Another technique for distributing hydrophobic compounds through a hydrophilic colloid solution or dispersion (which is subsequently coated and dried to yield a solid hydrophilic colloid layer in which the hydrophobic compounds are dispersed) is described in U.S. Pat. Nos. 2,304,940 and 2,322,027. This technique involves initially forming a solution by dissolving the hydrophobic compound(s) in oils or higher boiling solvents and then dispersing the resulting oily solution into the hydrophilic colloid solution or dispersion. Variations of this general technique have included the use of a lower molecular weight auxiliary solvent such as ethyl acetate or a lower molecular weight ketone to aid in the solubilization of the hydrophobe in the oily solvent, like for example the method described in U.S. Pat. No. 2,801,171. For the manufacture of "color" (dye image) photographic elements containing, incorporated therein, ballasted color-forming coupler compounds, techniques involving the use of such oily, higher boiling solvents, so-called "coupler solvents" have become widespread commercially. However, the techniques of dispersion of such coupler solutions require a high energy milling step (to obtain the desired degree of dispersion and particle size), and this usually results in some undesired degradation of some of the ingredients in the milled product. Also, such a milling step is both time consuming and expensive. Thus for many years there has existed a need for an improved method for dispersing hydrophobic compounds, like ballasted color-forming couplers, uniformly through photographic emulsions and other hydrophilic colloid-containing dispersions and solutions, which method would at least obviate the necessity to use a high energy mill to prepare useful dispersions of the hydrophobic materials through the hydrophilic colloid-containing materials and layers.
The present invention comprises a use of certain polymeric latexes (latices) in conjunction with the dispersion of hydrophobic materials into and through coatings or layers, such as those found in photographic elements, which use differs substantially from prior uses of latexes.
Some prior uses of latexes, for example, involved the use of latexes merely as one source for a polymeric ingredient in the coating layers. Conventional latexes have often been simply blended into a photographic emulsion which usually contained gelatin, silver halide and the usual photographic addenda. When organic solvents and hydrophobic compounds such as color-forming couplers were used heretofore in conjunction with synthetic polymers in the manufacture of coating compositions, often both the hydrophobe and the polymer were dissolved in the solvent prior to being formulated into the remainder of the coating compositions. (See for example, U.S. Pat. Nos. 3,518,088; 2,269,158; and 3,619,195).
In Tong U.S. Pat. No. 2,772,163, issued Nov. 27, 1956, the utility of latexes in distributing color-forming couplers in photographic element hydrophilic colloid layers was recognized. Tong discovered that couplers of limited solubility could be dissolved in alkali and, optionally, alcohol. By mixing the dissolved coupler containing alkali solution with a latex followed by neutralizing the alkali the coupler is precipitated from solution so that finally dispersed particles of the coupler are formed. The particles are, however, more finely dispersed than when the latex particles are absent.
Millikan U.S. Pat. No. 3,418,127, issued Dec. 24, 1968, discloses a method of finely dispersing a mixture of fluorescent compounds in a latex. This is accomplished by mixing the fluors, the monomeric precursors for the latex polymer particles and a polymerization initiator. Upon polymerization the combination of fluors is highly dispersed within the resulting latex. While this approach for dispersing fluors is potentially useful for dispersing other types of photographic hydrophobes, the dispersing process is not applicable to dispersing hydrophobes in preformed latexes and is limited to hydrophobes and hydrophobe concentrations which are compatible and attainable, respectively, with the polymerization techniques disclosed.